1 Radiometer systems Chris Allen ([email protected]) Course website URL people.eecs.ku.edu/~callen/823/EECS823.htm

1

Radiometer systems

Chris Allen ([email protected])

Course website URL

people.eecs.ku.edu/~callen/823/EECS823.htm

2

OutlineEquivalent noise temperature

– Characterization of noise

– Noise of a cascaded system

– Noise characterization of an attenuator

– Equivalent-system noise power at the antenna terminals

– Equivalent noise temperature of a superheterodyne receiver

Radiometer operation– Effects of gain variations

– Dicke radiometer

Examples of developed radiometers

Synthetic-aperture radiometers

3

Radiometer systemsA radiometer is a very sensitive microwave receiver that outputs a voltage, Vout, that is related to the antenna

temperature, TA.

Based on the output voltage, the radiometer estimates TA

with finite uncertainty, T, which is referred to as the radiometer’s sensitivity or radiometric resolution.

Radiometric resolution is a key parameter that characterizes the radiometer’s performance.

An understanding of the factors affecting radiometer’s performance characteristics requires an understanding of noise, radiometer design, and calibration techniques.

4

Equivalent noise temperatureIn any conductor with a temperature above absolute zero, the electrons move randomly with their kinetic energy proportional to the temperature T.

The randomly moving electrons produce a fluctuating voltage Vn called thermal noise, Johnson noise, or Nyquist

noise.Other kinds of noise include quantum noise (related to the discrete nature of electron energy), shot noise (fluctuations due to discrete nature of current flow in electronic devices), and flicker noise (also known as pink noise or 1/f noise) that arises from surface irregularities in cathodes and semiconductors.

Thermal noise is characterized with a zero mean, Vn = 0,

and is has equal power content at all frequencies, hence it is often called white noise.

5

Equivalent noise temperatureFor a conductor with resistance R connected to an ideal filter with bandwidth B, the output noise power Pn is

where k is Boltzmann’s constant (1.38 10-23 J K-1), T is the absolute temperature (K).

The thermal noise power delivered by a noisy resistor at temperature T is found by replacing the noisy resistor with a noise-free resistor and a voltage generator Vrms.

The reactance X represents the resistor’s inductive and capacitiveaspects.

BTkPn

BTkR4tVV 2n

2rms

6

Why V2rms = 4 R k T B ?

Experimental studies in 1928 by J.B. Johnson and theoretical studies by H. Nyquist of Bell Laboratories showed that the mean-squared voltage from a metallic resistor is

Therefore when attached to a matched load, RL = R,

the voltage developed across the load is cut in half (voltage divider) and therefore the mean-squared voltage is reduced by a factor of 4

The power transferred to the matched load is

BTkR4tV2n

BTkRtV L2L

BTk

R

tVP

L

2L

n

Open

circuit

voltage

7

Is it really white noise Quantum theory shows that the mean square spectral density of thermal noise is

At “low” frequencies this expression reduces to

For most applications where To 290 K (63 ºF)

[which conveniently simplifies the numerical work as kTo = 4 x 10-21 J]

If the resistance is at To then Gv(f) is essentially constant for

This conclusion holds even for cryogenic temperatures (T 0.001 To)

HzV1e

fhR2fG 2

Tkfhv

h

Tkffor

Tk2

fh1TkR2fG v

THz1orHz10hTk1.0f 12o

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Equivalent noise temperatureNow replace the noisy resistor with an antenna with radiometric antenna temperature TA′.

TA′ is the antenna weighted apparent temperature that includes the

self-emission of the lossy antenna.

If the same average power is delivered into the matched load, then we can relate TA′ to the thermodynamic

temperature T of the resistor.

9

Characterization of noiseNow consider the added noise of a linear two-port device (e.g., amplifier, filter, attenuator, cable).

Input to this device is a signal, Psi, and noise, Pni.

Output from this device is a signal, Pso, and noise, Pno.

The signal-to-noise ratio, SNR, can be determined at the input as well as the output.

For an ideal component (e.g., ideal amplifier), the input signal and noise would both be amplified by the same gain resulting in an SNRout = SNRin.

However noise sources within the component will cause SNRout < SNRin.

nosooutnisiin PPSNRPPSNR

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Characterization of noise

The ratio of SNRin to SNRout is called the noise figure, F.

where T0 is defined to be 290 K.

Therefore F 1 and is may be expressed in decibels

noso

nisioutin PP

PPSNRSNRF

BTk

PBTkG

G

1

P

P

P

PF

0

no0

ni

no

so

si

BTkG

P1F

0

no

Flog10dBF 10

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Characterization of noiseThe noise added by the two-port component, Pno, is

and the total output noise power, Pno, is

The two-port device may be treated as an ideal (noise-free) device with an external noise source added to the device’s input.

BTkG)1F(P 0no

ni0no

00no

PGFBTkGFP

BTkG)1F(BTkGP

12

Characterization of noiseAlternatively the noise added by the two-port component, Pno, can be expressed in terms of an equivalent input

noise temperature, TE, such that

Therefore if the noise power input to the device is characterized in terms of its noise temperature, TI, then the

output noise temperature is TI + TE such that

BTkGBTkG)1F(P E0no

0E T)1F(T

BTTkGP EIno

13

Characterization of noise

14

Characterization of noiseExample cases

The NEXRAD (WSR-88D) weather radar has an effective receiver temperature of 450 K.

Therefore its receiver noise figure is F=1+TE/T0

so F = 2.55 or 4 dB.

The SKYLAB RADSCAT radiometer had a 7.1-dB receiver noise figure.

Therefore its effective receiver temperature was where F = 5.1 so that TE = 1200 K.

0E T)1F(T

15

Noise of a cascaded systemNow consider a system composed of two components (systems) in cascade (i.e., connected in series).

Assuming B1 = B2, it can be shown that

1

21 G

1FFF

1

2E1EE G

TTT

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Noise of a cascaded systemSo the gain of the first component reduces the impact of the second component’s noise characteristics on the overall system’s noise performance.

For a system with N components or subsystems cascaded

Clearly if components with “large” gain values are placed nearest the input, their noise characteristics will determine the system’s noise characteristics.

1N21

N

21

3

1

21 GGG

1F

GG

1F

G

1FFF

1N21

EN

21

3E

1

2E1EE GGG

T

GG

T

G

TTT

17

Noise characterization of an attenuatorNext consider an attenuator with a physical temperature Tp

and a loss factor L.

For L > 1, Pout < Pin.

Examples include a lossy cable, a filter with insertion loss, or an RF switch.

For L = 1 dB, L =1.26; if L = 1.5 dB, then Pout = 70.8% of Pin

The noise figure, F, of an attenuator is

where T0 = 290 K. If TP = T0, then F = L.

Similarly the equivalent temperature is

outin PPG1L

0P TT)1L(1F

0E T)1L(T

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Noise of a cascaded systemExample

Given:

F1 = 2

L = 3 dB, F2 = 2

G3 = 30 dB, F3 = 5

Find Fsys for G1 = (a) 5 dB, (b) 10 dB, (c) 30 dB

(a) G1 = 5 dB or 3, Fsys = 2 + (2-1)3 + (5-1)(3/2) = 5

(b) G1 = 10 dB or 10, Fsys = 2 + (2-1)10 + (5-1)5 = 2.9

(c) G1 = 30 dB or 1000, Fsys = 2 + (2-1)1000 + (5-1)500 = 2.01

Therefore, if G1 » L Fsys F1

Preamp Limiter Amplifier

G1

F1

G2 = 1/LF2 = L

G3

F3

Assume B1 = B2 = B3

TP (atten) = T0

LG

1F

G

1FFF

1

3

1

21sys

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Noise of a cascaded systemExample

Given:

F1 = 2, TE1 = 290 K

L = 3 dB, F2 = 2, TE2 = 290 K

G3 = 30 dB, F3 = 5, TE3 = 1160 K

Find TE for G1 = (a) 5 dB, (b) 10 dB, (c) 30 dB

(a) G1 = 5 dB or 3, TE = 290 + 2903 + 1160(3/2) = 1160 K

(b) G1 = 10 dB or 10, TE = 290 + 29010 + 11605 = 551 K

(c) G1 = 30 dB or 1000, TE = 290 + 2901000 + 1160500 = 293 K

Therefore, if G1 » L TE TE1

LG

T

G

TTT

1

3E

1

2E1EE

G1

F1

TE1

G2 = 1/LF2 = L

TE2

G3

F3

TE3

Assume B1 = B2 = B3

TP (atten) = T0

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Equivalent-system noise power at the antenna terminals

Losses in the antenna (radiation efficiency, l < 1) add

noise to the antenna’s output

And transmission-line losses raise the receiver’s equivalent input noise temperature

PAA T1TT ll

RECPREC TLT1LT

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The overall system input noise temperature, TSYS, is

Assuming the antenna and transmission line are at the same temperature, TP.

Recall that TA, the desired parameter, must be estimated from

PSYS

Estimation of TA from PSYS requires accuracy and precision

Accuracy: conformity of a measured value to its actual value without bias

Bias: a systematic deviation of a value from a reference value

Precision: ability to produce the same value on repeated independent observations

RECPPASYS TLT1LT1TT ll

BTkP SYSSYS

22


Calibration provides a means to achieve the desired accuracy.

A linear transfer function relates Vout to TA′

Find a and b using two different calibration temperatures, TCAL

thus removing any systematic biases.

Why is Vout TA′? Isn’t TA′ P instead of V?

bVaT outA

23


Vout is the output of a square-law detector

( )2Vin Vout

2inout VV

inout PV

inA PT

Aout TV so

24


Precision relates to T, the radiometric resolution which is the smallest detectable change in TA′.

Determination of T requires an understanding of the signal’s statistical properties.

Consider the total-power radiometer

Total-power radiometer block diagram

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The total system input noise power is PSYS where

and

The average power at the IF amplifier output, PIF, is

BTkPPP SYSRECASYS

RECASYS TTT

BTkGP SYSIF

PIF

26


27


The instantaneous IF voltage, VIF(t), has the characteristics

of thermal noise, i.e., Gaussian probability distribution and a zero mean and standard deviation, while the envelope of VIF(t) has a Rayleigh distribution

For a Rayleigh distribution, the mean value of Ve

2 is

0Vfor,0

0Vfor,eV

Vp

e

e2V

2e

e

22e

22e 2V

28


After the square-law detector we have Vd = Cd Ve2 where Cd

is the power-sensitivity constant of the square-law detector (V W-1) and Vd is the output voltage.

Vd will have an exponential distribution

with the property that the variance of Vd is d, which leads

to d / Vd = 1.

This is significant since the variance is the uncertainty, so the measured uncertainty = the mean value.

dd VV

dd e

V

1Vp

SYSdIFdd TBkGCPCV

29


So without additional signal processing, a measured value of 250 K would have an uncertainty of ±250 K! (unacceptable)

To reduce the measurement uncertainty, multiple independent samples of the signal are averaged.

The low-pass filter which acts as an integrator, performs this averaging.

Assuming the signal is constant over the averaging interval, the mean value should remain unchanged while the variance is reduced.

Here B is the RF bandwidth and is the integration time (s) constant which is related to the low-pass filter’s bandwidth by BLPF 1/(2 ), Hz.

B

1

VV 2d

2d

2out

2out

B

1

Vout

out

30


So the ratio of the measurement uncertainty to the measured value is

and since TSYS = TA′ + TREC′

So the measurement uncertainty due to noise processes, TN, is

B

1

T

T

SYS

SYS

B

TTT RECA

SYS

B

TT SYS

N

31


Besides uncertainties due to noise processes, variations in the receiver gain will also introduce measurement uncertainty, TG.

Since Vd = CdG k B TSYS, variations in G will cause

variations in the detected signal, Vd.

The uncertainty resulting from gain variations is

where GS is the average system power gain and GS is the

RMS variation of the power gain.

The magnitude of GS can be reduced (though not to 0) by

periodically calibrating the radiometer output voltage when inputting a known noise source.

SSSYSG GGTT

32


The combination of these two uncertainty terms, TN and

TG , produce an total uncertainty, T through a root-sum-

square (RSS) process

or

2G

2N TTT

2

S

SSYS G

G

B

1TT

33


ExampleRadiometer center frequency, f = 1.4 GHz

Bandwidth, B = 100 MHz

Receiver noise temperature, TREC′ = 600 K (F = 3 or 4.9 dB)

Antenna temperature, TA′ = 300 K

Low-pass filter bandwidth, BLPF = 50 Hz ( = 10 ms)

System gain, GS = 50 dB ± 0.044 dB over 10 ms interval

Find T and determine which factor (noise or gain variation) dominates.

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Example (cont.)

Find TN: TSYS = TREC′ + TA ′ = 600 + 300 = 900 K

TN = TSYS (B )- ½ = 900 (108 ·10-2)- ½ = 0.9 K

Find TG: GS = 50 dB or 100,000

GS + GS = 50.044 dB or 101,018

GS GS = 49.956 dB or 98,992

so |GS| 1013

GS / GS = 1013/100,000 = 1%

TG = TSYS (GS / GS) = 9 K

T = [TN2 + TG

2] ½ = [(0.9)2 + (9)2]½ = 9.05 K

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Example (cont.)

T = 9.05 K and is dominated by TG (9K)

To reduce the affect of TG so that it is comparable to TN

requires that GS/GS = 0.001 or G = 100

so that GS = 100,000 ± 100

or GS = 50 dB ± 0.004 dB over a 10 ms interval

A study of GS properties shows the following:

GS varies as 1/f

for f 1 kHz, GS 0

worst for f < 1 Hz

Therefore we want < 1 ms to make TG small.

However to make TN small we want > 1 ms.

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Dicke radiometerThe Dicke radiometer solves the dilemma concerning .

Synchronous switching and detection permits

fs > 1 kHz with » 1 ms

37

Dicke radiometerThe Dicke radiometer alternates between two configurations, one where it samples the TA′ and the other where it samples

a reference load, TREF.

When the switch connects the antenna to the receiver

When the switch connects the reference load to the receiver

These are combined in the synchronous detector to form

2t0for,TTBkGCV SRECAdANTd

SSRECREFdREFd t2for,TTBkGCV

REFdANTdSYNd VV2

1V

REFAdSYNd TTBkGC2

1V Notice that TREC′ cancels out

The ½ term is due to the dwell time in each switch position

38

Dicke radiometerVSYN is integrated in the low-pass filter to yield VOUT

where GS is a constant representing the radiometer’s

transfer characteristics.

The low-pass filter not only integrates the signal but also rejects the components at fs and its harmonics that are

introduced by the square-wave modulation.

This requires that fs 2 BLPF.

Under these conditions VOUT (TA′ – TREF) and is

independent of TREC′.

REFASOUT TTG2

1V

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Dicke radiometerTo evaluate the Dicke radiometer’s sensitivity T begin with

Three components contribute to T

Gain variations

Noise uncertainty in TA′

Noise uncertainty in TREF

So that

RECREFRECASOUT TTTTG2

1V

SSREFAG GGTTT

B

TT2T RECA

ANTN

B

TT2T RECREF

REFN

2REFN

2ANTN

2G TTTT

40

Dicke radiometerNow it might appear that we’re no better off than we were with the total-power radiometer (and maybe worse off with 3 terms comprising T now)

However notice the difference in TG

total power

Dicke

In the Dicke radiometer this term (which dominated T in the total-power radiometer) is significantly smaller.

SSREFAG GGTTT

2REFN

2ANTN

2G TTTT

SSRECAG GGTTT

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Dicke radiometerExample

Bandwidth, B = 100 MHz

Low-pass filter bandwidth, BLPF = 0.5 Hz ( = 1 s)

Receiver noise temperature, TREC′ = 700 K (F = 3.4 or 5.3 dB)

Reference temperature, TREF = 300 K

System gain variations, GS/ GS = 1%

When viewing a 0-K target (TA′ ~ 0 K)

T (total-power) = 7.0 K [TG = 7.0 K, TN ANT = 0.07 K]

T (Dicke) = 3.0 K [TG = 3.0 K, TN ANT = 0.1 K, TN REF = 0.14 K]

When viewing a 300-K target (TA′ ~ 300 K)

T (total-power) = 10.0 K [TG = 10.0 K, TN ANT = 0.1 K]

T (Dicke) = 0.2 K [TG = 0.0 K, TN ANT = 0.14 K, TN REF = 0.14 K]

(Note that TREF – TA′ = 0, a balanced condition)

42

Balanced Dicke radiometerOperating in a balanced mode requires adjusting TREF.

Using a “cold” noise source and a variable attenuator (TP = 290 K) permits balanced mode operation for a wide range of targets.

43

Radiometer calibrationAs mentioned earlier calibration is needed for accurate measurements.

For ground-based systems warm calibration targets are abundant so the cold target calibration poses a challenge.

Shown here is a cryoloadthat is useful for periodiccalibration of modest-sized antennas.

When filled with liquidnitrogen this cold target hasa radiometric temperature ofaround 77 K.

44

Radiometer calibrationAnother approach that may accommodate modest-sized as well as medium sized antennas is the bucket method that uses the naturally cold sky as a cold target.

As seen previously, TSKY is dependent on the operating

frequency and on the weather conditions.

For clear-sky conditions the zenithsky temperature will rangebetween about 5 and 120 K.

Using this calibration technique the antenna efficiency, l, can be

estimated.

45

Radiometer calibrationCalibration of spaceborne radiometer systems requires features that enable periodic calibration during flight.

46

Scanning Multichannel Microwave Radiometer (SMMR)

Flew on two platforms

SEASAT (28 June 1978 to 10 October 1978)

NIMBUS-7 (26 October 1978 to 20 August 1987)

Intended to obtain ocean circulation parameters such as sea surface temperatures, low altitude winds, water vapor and cloud liquid water content on an all-weather basis.

Ten channels: five frequencies, dual polarized

Mechanically scanned antenna

47


48


Antenna system• Shared 79-cm diameter offset parabolic reflector used by all channels

• Mechanical scanning, 42 off-nadir look angle, ±25 azimuth angle range, scan period 4.096 s

• Provided constant 50.3 incidence angle across 780-km swath

Offset-fed parabolic reflector geometry

49


Antenna scan characteristics

50

Radiometer antennasAntennas pose a challenge in some radiometer applications.

Spatial resolution (x, y) is set by the antenna beamwidth.

Consider an antenna with L 20 or = 3.

A spot diameter at nadir of 520 m results at aircraft altitudes (10 km).

At spacecraft altitudes (700 km) a spot diameter at nadir of 37 km is produced.

To achieve a finer resolution requires a smaller beamwidth, i.e., a larger antenna.

In addition, mechanical beamsteering limits the scan rate and reliability.

51

Synthetic-aperture radiometersFrom antenna array theory we know that arraying separate apertures produces a radiation pattern that is the product of the pattern of the individual element and the array pattern.

However grating lobes result from large element spacings.Tapers (weighting functions) are used for sidelobe suppression.

52

Synthetic-aperture radiometersThe underlying idea of the synthetic-aperture radiometer is that with an array of receiving elements, multiple beams can be formed simultaneously to image a swath.

This is accomplished by cross-correlated signals from a pair of antennas with overlapping fields of view.

The approximate null-to-null beamwidth () is

Where D is the maximum antenna spacing.

radians,D2

From Ruf CS; Swift CT; Tanner AB; LeVine DM; “Interferometric aperture synthesis,” IEEE Trans. Geoscience and Remote Sensing, 26(5), pp 597-611, 1988.

53

Synthetic-aperture radiometersThe Electronically Steered Thinned Array Radiometer (ESTAR) is a hybrid which uses real aperture for along-track resolution and aperture synthesis for cross-track resolution.

From Elachi C; van Zyl J; Introduction to the physics and techniques of remote sensing , Wiley, 2006.

54

Synthetic-aperture radiometersHYDROSTAR conceptFor mapping soil moisture and sea salinity

55

Synthetic-aperture radiometersSMOS concept (Soil Moisture and Ocean Salinity)Aperture synthesis in two dimensions

56

SMOS satelliteSMOS satellite

– 1,451-pound, $464M

– launched on 1 Nov 2009

– altitude of 465 miles to 476 miles

– inclination of 98.4°

SMOS carries as the L-band MIRAS radiometer that uses a Y-shaped antenna resembling the rotors of a helicopter, is a first-of-a-kind payload comprising 69 individual antennas strung together in an inferometer-like array to maximize the sensor's sensitivity.

A Rockot launch vehicle blasts off on Nov. 1, 2009 with the Europe's Proba 2 and Soil Moisture Observation Satellite (SMOS) from the Plesetsk Cosmodrome in northern Russia.

57

MIRAS radiometer3-year mission is to investigate Earth's water cycle

– measures soil moisture, 4% accuracy, 30-mile resolution

– measures ocean salt concentration, 120-mile resolution

Soil moisture is a key factor in determining humidity in the atmosphere and the formation of precipitation. These data will also aid researchers studying plant growth and vegetation distributions.

Ocean salinity maps reveal how the atmosphere and oceans interact by providing new insights on ocean circulation, a major driver of world climate.

An artist's concept of the Soil and

Moisture Observation Satellite (SMOS)

satellite with deployed solar arrays and

instrument

58

MIRAS radiometerThe MIRAS radiometer operates in the L-band of the electromagnetic spectrum in a band (1400-1427 MHz) reserved (by the International Telecommunications Union) for space research, radio astronomy and a radio communication service between Earth stations and space, known as the Earth Exploration Satellite Service.

Since its 2009 launch and system check out, project scientists noticed that over certain areas the MIRAS radiometer data were badly contaminated by radio-frequency interference (RFI).

The unwanted signals have mainly come from TV transmitters, radio links and networks such as security systems. Terrestrial radars appear to also cause some problems.

SMOS data revealed that there were many instances of other signals within this protected band, particularly in southern Europe, Asia, the Middle East and some coastal zones.

59

MIRAS radiometer data

March 2010 SMOS image from, over Spain showing contamination by unwanted transmissions from various radio systems.

Google Earth image

60

MIRAS radiometer data

July 2010 image following cooperation between ESA and the National Spectrum Authority, SMOS data over Spain showing far less RFI contamination.

Google Earth image

61

RFI mitigation in radiometersIn their paper [1] “RFI detection and mitigation for microwave radiometry with an agile digital detector,” the authors state that:– A new type of radiometer has been developed that incorporates an “agile digital

detector” (ADD)

– This radiometer is capable of identifying high and low levels of radio frequency interference (RFI)

– The effects of this RFI on measured brightness temperature can be reduced or eliminated

High-level RFI has a high SNR and a narrow bandwidth• communication source

Low-level RFI has a low SNR and a broad bandwidth or is persistent• radar

The agile digital detector exploits– Difference of signal probability density function (pdf)

• noise Gaussian pdf, sinusoid non-Gaussian pdf

– Spectrally localized RFI

[1[ Ruf CS; Gross SM; Misra S; “RFI detection and mitigation for microwave radiometry with an agile digital detector,” IEEE Transactions on Geoscience and Remote Sensing, 44 (3), pp. 694-706, 2006.

62

RFI mitigation in radiometersSignal pdf analysis– Conventional radiometers use analog square-law detector 2nd moment

– With a digitized waveform the 2nd moment can be computed thus replicating conventional signal processing;but it can also compute the higher-order moments, e.g., 4th moment

– The ratio between the 4th moment and the 2nd moment is stable for Gaussian signals and “quite responsive to low-level RFI

– Subbands filtering allows isolation of subband containing RFI

– Channel crosscorrellation is possible permitting cross-polarization analysis

63

RFI mitigation in radiometersFunctional block diagram of ADD signal processor

64

RFI mitigation in radiometersADD filter bank characteristics

Predicted transfer function of ADD filter banks using a Kaiser = 3.2 window with 47 taps and nine-bit signed coefficients.

65

RFI mitigation in radiometersSample ADD measurement of the pdf of the signal entering the radiometer while viewing a continuous sine wave at 1412 MHz (centered on subband #4) added to background radiometric emission with the antenna pointed toward cold sky. The pdf is shown for four of the eight frequency subbands.

ADD measurements over 1 min of the continuous sine wave plus background cold sky scene as above. Brightness temperature is shown versus time forfour of the eight frequency subbands. The RFI is centered in subband #4, which exhibits a brightness temperature of ~2000 K. Subband #3 has 50–60-K levels of RFI due to the out-of-band rejection limitations of their filters. Subbands #1–2 are essentially RFI free.

66

RFI mitigation in radiometers

ADD measurements as before. The normalized ratio between the fourth moment and the square of the second moment of the signal is shown for four of the eight frequency subbands.A normalized ratio of unity indicates that the signal has a Gaussian distributed amplitude consistent with natural thermal emission.The ratio for subband #4 is approximately 0.6 due to the strong sine wave present.Subband #3 also has small nonunity ratios as a result of the RFI. The other subbands are RFI free.Note the scale change for subbands #3–4.

Documents

1 Radiometer systems Chris Allen ([email protected]) Course website URL people.eecs.ku.edu/~callen/823/EECS823.htm